Balzaccom is right: you don’t recover as fast, so the key is to try to avoid doing things you have to recover from. (I was circling around it with the “hero trek” comment, but I like his more direct approach.) One thing I avoid is a heavy pack.

In my own case, I’ve found that 15 mile days are beyond me, especially if much elevation gain or loss is involved. I now typically plan for 8-10 mile days instead, in gentler terrain; I also don’t plan for more than 2 or 3 nights out - the cumulative wear and tear takes a few days to rest up. I’ve also found that I don’t react as well to the cold, so I stay home when the predicted lows are below 35; I also avoid prolonged rain or snow - mostly because I don’t enjoy it any more, and I’m old enough I don’t have to prove I can do it.